U.S. patent number 5,821,917 [Application Number 08/504,433] was granted by the patent office on 1998-10-13 for system and method to compensate for the effects of aging of the phosphors and faceplate upon color accuracy in a cathode ray tube.
This patent grant is currently assigned to Apple Computer, Inc.. Invention is credited to Richard D. Cappels.
United States Patent |
5,821,917 |
Cappels |
October 13, 1998 |
System and method to compensate for the effects of aging of the
phosphors and faceplate upon color accuracy in a cathode ray
tube
Abstract
A system and method of compensating for the effects of aging of
phosphors and faceplate upon color accuracy in cathode ray tubes,
wherein beam current measurements are made upon individual cathodes
of a cathode ray tube to sample the individual beam currents at
periodic intervals. The sum-totals of the individual beam current
measurements are then stored in a non-volatile memory location.
Correction factors are calculated for both luminous efficiency
degradation and for deviations in hue, based on the stored
sum-total beam current measurements in combination with
empirically-derived formulae. These correction factors are then
used to calculate corrected tristimulus values X, Y, and Z. The
corrected tristimulus values are used to calculate the amount of
beam current necessary to compensate for color degradation of the
cathode ray tube. Finally, the respective gains of the video
amplifiers are adjusted to achieve the amount of beam current
necessary to compensate for the effects of aging of the cathode ray
tube.
Inventors: |
Cappels; Richard D. (San Jose,
CA) |
Assignee: |
Apple Computer, Inc.
(Cupertino, CA)
|
Family
ID: |
24006239 |
Appl.
No.: |
08/504,433 |
Filed: |
July 20, 1995 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
36349 |
Mar 24, 1993 |
5512961 |
|
|
|
Current U.S.
Class: |
345/589; 345/22;
348/173; 345/204; 348/E9.041; 348/E9.051; 348/E17.005 |
Current CPC
Class: |
H04N
9/73 (20130101); H04N 9/645 (20130101); H04N
17/04 (20130101); G09G 2320/043 (20130101) |
Current International
Class: |
H04N
9/73 (20060101); H04N 17/04 (20060101); H04N
9/64 (20060101); G09G 005/02 () |
Field of
Search: |
;345/204,207,211,212,213,214,117,22,150,151,185,203
;348/658,173,191,175,176,189,655,656,657,675,679 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0313795 |
|
May 1989 |
|
EP |
|
7107508 |
|
Apr 1995 |
|
JP |
|
2169773 |
|
Jul 1986 |
|
GB |
|
9312616 |
|
Jun 1993 |
|
WO |
|
9422270 |
|
Sep 1994 |
|
WO |
|
Other References
DL. MacAdam, Color Measurement, Theme and Variation, 2nd Ed., pp.
9-21. .
Steve Roth, Managing Color, Jan. 1993 -Macworld, pp. 148, 150-155.
.
Bruce Fraser, Getting Color in Sync, Mar., 1993 -MacUser, pp.
165-167; 178-181. .
Hitoshi Takaoka, Yoshinori Ogata, "A New Input Modulation Method
For Generating Expected Colors On A CRT Monitor," 1991, pp.
57-60..
|
Primary Examiner: Wu; Xiao
Attorney, Agent or Firm: Carr & Ferrell LLP Koerner;
Gregory J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present invention is a continuation-in-part of U.S. patent
application Ser. No. 08/036,349 filed Mar. 24, 1993, now U.S. Pat.
No. 5,512,961 by Richard D. Cappels, Sr., and entitled "Method And
System Of Achieving Accurate White Point Setting Of A CRT Display,"
which is hereby incorporated by reference.
Claims
What is claimed is:
1. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement; and
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements.
2. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement said measuring circuit comprising
an internal processor for generating color digital video
signals,
a display controller, connected to the internal processor for
converting the digital video signals into analog video signals,
video amplifiers connected to the display controller for receiving,
amplifying, and supplying amplified analog video signals to the
cathode ray tube,
current samplers coupled between the video amplifiers and the
cathode ray tube, for sampling the video signals and generating
analog beam current measurements,
an analog-to-digital converter coupled between the current samplers
and the internal processor, for converting the analog beam current
measurements into digital beam current measurements, and
non-volatile memory connected to the internal processor, for
storing the sum-total beam current measurements;
and
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements.
3. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement, said measuring circuit comprising
an internal processor for generating color digital video
signals,
display controller, connected to the internal processor for
converting the digital video signals into analog video signals,
video amplifiers connected to the display controller for receiving,
amplifying, and supplying amplified analog video signals to the
cathode ray tube,
current samplers coupled between the video amplifiers and the
cathode ray tube, for sampling the video signals and generating
analog beam current measurements,
an analog-to-digital converter coupled between the current samplers
and the internal processor, for converting the analog beam current
measurements into digital beam current measurements, and
non-volatile memory connected to the internal processor, for
storing the sum-total beam current measurements, said non-volatile
memory being an Electrically Erasable Programmable Read-Only-Memory
(EEPROM);
and
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements.
4. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement, said measuring circuit comprising
an internal processor for generating color digital video
signals,
a display controller, connected to the internal processor for
converting the digital video signals into analog video signals,
video amplifiers connected to the display controller for receiving,
amplifying, and supplying amplified analog video signals to the
cathode ray tube,
current samplers coupled between the video amplifiers and the
cathode ray tube, for sampling the video signals and generating
analog beam current measurements,
an analog-to-digital converter coupled between the current samplers
and the internal processor, for converting the analog beam current
measurements into digital beam current measurements, and
non-volatile memory connected to the internal processor, for
storing the sum-total beam current measurements;
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements; and
a video monitor cabinet containing the internal processor, display
controller, video amplifiers, current samplers, cathode ray tube,
analog-to-digital converter, and non-volatile memory.
5. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement, said measuring circuit comprising
an internal processor for generating color digital video
signals,
a display controller, connected to the internal processor for
converting the digital video signals into analog video signals,
video amplifiers connected to the display controller for receiving,
amplifying, and supplying amplified analog video signals to the
cathode ray tube,
current samplers coupled between the video amplifiers and the
cathode ray tube, for sampling the video signals and generating
analog beam current measurements,
an analog-to-digital converter coupled between the current samplers
and the internal processor, for converting the analog beam current
measurements into digital beam current measurements, and
non-volatile memory connected to the internal processor, for
storing the sum-total beam current measurements;
and
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements, said compensating circuit comprising host processor
means, connected to the internal processor means, for controlling
the internal processor means to compensate for the effects of aging
upon color accuracy in the cathode ray tube.
6. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube comprising:
a timer for generating a beam current measurement command at
periodic intervals;
a measuring circuit responsive to the timer, for making a sum-total
beam current measurement, said measuring circuit comprising
an internal processor for generating color digital video
signals,
a display controller, connected to the internal processor for
converting the digital video signals into analog video signals,
video amplifiers connected to the display controller for receiving,
amplifying, and supplying amplified analog video signals to the
cathode ray tube, said amplified analog video signals comprising
separate red, green, and blue color signals,
current samplers coupled between the video amplifiers and the
cathode ray tube, for sampling the video signals and generating
analog beam current measurements,
an analog-to-digital converter coupled between the current samplers
and the internal processor, for converting the analog beam current
measurements into digital beam current measurements, and
non-volatile memory connected to the internal processor, for
storing the sum-total beam current measurements;
and
a compensation circuit coupled to the cathode ray tube, for
adjusting beam current in response to the sum-total beam current
measurements.
7. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement.
8. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube, said step of calculating a calculated sum-total
beam current measurement comprising the steps of
providing a non-volatile memory for storing the sum-total beam
current measurement for each of three primary color video
amplifiers,
storing into the non-volatile memory an initial value of zero for
the sum-total beam current measurement for each of the three
primary colors,
adding the periodic beam current measurement for each video
amplifier to the sum-total beam current measurement value
previously stored in the non-volatile memory to form a new
sum-total beam current measurement for each video amplifier,
and
storing the new sum-total beam current measurement in non-volatile
memory to update the sum-total beam current measurement for each
video amplifier;
and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement.
9. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v',
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
10. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v', said step of calculating a luminous efficiency
degradation factor calculating luminous efficiency degradation
factor .eta. for red and green according to the formula ##EQU3##
wherein .eta..sub.(start) is the initial efficiency of a red or
green phosphor, C is the phosphor burn sensitivity parameter, and N
is the dosage in Coulombs of electrons deposited per square
centimeter of phosphor,
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
11. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v', said step of calculating a luminous efficiency
degradation factor calculates a luminous efficiency degradation
factor .eta..sub.Blue for blue according to the formula ##EQU4##
wherein .eta..sub.Blue(start) is the initial efficiency of a blue
phosphor, C is the phosphor burn sensitivity parameter, N is the
dosage in Coulombs of electrons deposited per square centimeter of
phosphor, S is the shape of a blue component degradation
modification curve, and L is the amplitude of a blue component
degradation modification curve,
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
12. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v', said step of calculating hue deviation factors
comprising the steps of
dividing the initial individual red, green, and blue tristimulus
values X, Y, and Z by each of the beam current values to produce
normalized initial tristimulus values,
converting the normalized initial tristimulus values into
equivalent color coordinates u', v', and normalized initial Y,
calculating hue deviation factors .DELTA.u' and .DELTA.v,'
corresponding to shifts in color coordinates of the phosphor
emissions from the cathode ray tube, according to the formulae
.DELTA.u'=ND.sub.u' and .DELTA.v'=ND.sub.v', wherein N is the total
charge in Coulombs deposited on the phosphor, and D.sub.u' and
D.sub.v' are hue degradation factors for the u' and v' axes,
adding .DELTA.u' to u' to produce corrected u', and
adding .DELTA.v' to v' to produce corrected v',
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
13. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v', said step of calculating hue deviation factors
comprising the steps of
dividing the initial individual red, green, and blue tristimulus
values X, Y, and Z by each of the beam current values to produce
normalized initial tristimulus values,
converting the normalized initial tristimulus values into
equivalent color coordinates u', v', and normalized initial Y,
calculating hue deviation factors .DELTA.u' and .DELTA.v,'
corresponding to shifts in color coordinates of the phosphor
emissions from the cathode ray tube, according to the formulae
.DELTA.u'=ND.sub.u' and .DELTA.v'=ND.sub.v', wherein N is the total
charge in Coulombs deposited on the phosphor, and D.sub.u' and
D.sub.v' are hue degradation factors for the u' and v' axes,
adding .DELTA.u' to u' to produce corrected u', and
adding .DELTA.v' to v' to produce corrected v',
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors, said step of calculating corrected
tristimulus values comprising the further step of
multiplying the normalized initial tristimulus value Y by luminous
efficiency degradation factor .eta. to produce a normalized
corrected tristimulus value Y,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
14. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v', said step of calculating hue deviation factors
comprising the steps of
dividing the initial individual red, green, and blue tristimulus
values X, Y, and Z by each of the beam current values to produce
normalized initial tristimulus values,
converting the normalized initial tristimulus values into
equivalent color coordinates u', v', and normalized initial Y,
calculating hue deviation factors .DELTA.u' and .DELTA.v,'
corresponding to shifts in color coordinates of the phosphor
emissions from the cathode ray tube, according to the formulae
.DELTA.u'=ND.sub.u' and .DELTA.v'=ND.sub.v', wherein N is the total
charge in Coulombs deposited on the phosphor, and D.sub.u' and
D.sub.v' are hue degradation factors for the u' and v' axes,
adding .DELTA.u' to u' to produce corrected u', and
adding .DELTA.v' to v' to produce corrected v',
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors, said step of calculating corrected
tristimulus values comprising the further step of
multiplying the normalized initial tristimulus value Y by luminous
efficiency degradation factor .eta. to produce a normalized
corrected tristimulus value Y,
reconverting corrected color coordinates u' and v', and the
normalized corrected tristimulus value Y into equivalent normalized
corrected tristimulus values X, Y, and Z,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
15. A method of compensating for the effects of aging upon color
accuracy in a cathode ray tube, comprising the steps of:
generating a beam current in a cathode ray tube;
measuring the beam current of the cathode ray tube at periodic
intervals to produce a periodic beam current measurement;
calculating a calculated sum-total beam current measurement for the
cathode ray tube; and
compensating the beam current in the cathode ray tube as a function
of the calculated sum-total beam current measurement, said step of
compensating the beam current comprising the steps of
calculating, from the calculated sum-total beam current
measurements for respective video amplifiers and from
empirically-derived correction formulae, a luminous efficiency
degradation factor .eta. and hue deviation factors .DELTA.u' and
.DELTA.v',
calculating corrected tristimulus values X, Y and Z from initial
tristimulus values, the luminous efficiency degradation factor and
the hue deviation factors,
calculating, from the corrected tristimulus values X, Y and Z,
amounts of beam current predicted to compensate for degradations in
color accuracy,
generating a known-value screen display on the cathode ray tube as
a reference standard to calibrate the beam current of the video
amplifier, said step of generating a known-value screen display
generating a white screen display,
and
adjusting the gain of each video amplifier to produce beam current
which compensates for the degraded color accuracy of the cathode
ray tube.
16. A system to compensate for the effects of aging upon color
accuracy in a cathode ray tube, comprising:
means for generating a beam current in the cathode ray tube;
means for measuring the beam current in the cathode ray tube at
periodic intervals to produce a periodic beam current
measurement;
means for calculating a calculated sum-total beam current
measurements for the cathode ray tube; and
means for compensating the beam current as a function of the
calculated sum-total beam current measurement, to correct for
effects of aging upon color accuracy in the cathode ray tube.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to computer displays and more particularly
to a system and method of compensating for the detrimental effects
upon color accuracy resulting from aging of phosphors and
faceplates in cathode ray tubes (CRTs).
2. Discussion of the Prior Art
Maintaining color accuracy in computer monitors is of increasing
concern to many computer users as well computer manufacturers. The
proliferation of use of computers in applications where color
accuracy is critical makes faithful color reproduction more than
merely an aesthetically pleasing feature in a computer monitor.
Fields where color accuracy may be critical include medicine,
computer graphics, and engineering design work, for example.
Tristimulus values, as further explained in Color Measurement,
Theme and Variation, D. L. MacAdam, 2nd ed., Springer-Verlag, pp.
9-21, represent the amount of energy of light in overlapping bands
referred to as X, Y, and Z. The X, Y, and Z bands correspond to the
three channels of a model of human color vision known as the C.I.E.
standard of 1976, in which average observers perceive specific hues
according to the ratios of light energy in the three bands. The
tristimulus value ratio corresponds to a particular hue. Further,
the summed weighted energies of these three bands describe the
intensity or luminance of the light. Thus, a given set of
tristimulus values represents a specific hue at a specific
luminance.
The X, Y, and Z channels of the viewer's eyes are stimulated by
corresponding red, green and blue phosphors being bombarded with
electrons. The degree of stimulation of each of the three channels
depends upon the particular type of phosphors being bombarded with
electrons.
Various factors cause degradation of color produced by computer
monitors. One significant factor is aging of the cathode ray tube.
Over time, electron and ion bombardment changes the hue and
luminous efficiency of the light emitted from the phosphors used in
the face of a cathode ray tube. The mechanism of these changes is
thought to be the generation of non-emitting recombination centers
and/or the loss of activator centers due to changes in the state of
ionization of activator constituents. Each of the three primary
colors uses a respective phosphor having a different chemical
composition, hence having a different rate of deterioration and
aging, which also contributes to the total hue shift.
The rate of color degradation depends primarily upon beam current,
acceleration voltage, and CRT temperature. If the acceleration
voltage and temperature are held constant, as is typical in CRT
displays, then phosphor degradation in substantially a function of
the accumulated number of Coulombs of beam current passed through
the cathode and deposited on the phosphors of the CRT through its
history of operation.
Another significant contribution to color degradation is the aging
of the CRT's glass faceplate. High-energy electron and X-ray
bombardment changes the chemical structure of the faceplate glass
and unevenly reduces its transmission of light, dramatically more
at shorter wavelengths than at medium and longer wavelengths, thus
shifting the transmission of hues toward yellow. The faceplate's
rate of change in its transmission of light depends primarily upon
the total amount beam current and acceleration voltage over time.
If the acceleration voltage and image area are held constant, then
the CRT transparency change is substantially a function of the
total number of accumulated Coulombs of beam current directed at
the faceplate.
Attempts to compensate for color degradation in computer monitors
have conventionally taken several approaches. Referring to the
drawings, FIG. 1 shows a prior art system for compensating against
the effects of aging of phosphors 34 and faceplate 33 in a video
display 12. The computer monitor is provided with manual individual
color controls 13 to enable adjusting the red, green, and blue
video amplifier gains and the overall luminance level, and thus the
amounts of red, blue, and green on video display 12. This
"eyeballing" approach is inaccurate unless complemented with a
spectra-radiometer for measuring the tristimulus values of light
emitted from the video display 12. The spectra-radiometer 22 could
be replaced by another conventional light measuring device, such as
a photometer. To compensate for color degradation in video display
12, CPU 14 generates a known-chromaticity image such as a white
screen, which is displayed on the video display 12.
Spectra-radiometer 22 measures and displays the tristimulus values
of this image, and thus, the amounts of red, blue, and green on
video display 12 can be adjusted using color controls 13 located on
the video display 12. The color controls 13 are adjusted until the
tristimulus value readings on the spectra-radiometer 22 match the
expected chromaticity readings of the image being produced by CPU
14.
Another conventional approach supplies the user with an achromatic
card, or a series of colored cards, that serve as color standards
for matching to test patterns generated by the CPU 14. This system
is also somewhat inaccurate, and is time consuming.
Therefore, an improved system and method is needed to compensate
accurately for degradation of color in cathode ray tubes due to
phosphor and faceplate aging.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system and method are
disclosed for compensating for the effects of aging of the
phosphors and faceplate upon color accuracy in cathode ray tubes.
In the preferred embodiment of the present invention, an internal
processor generates and communicates a digital video signal to a
display controller. The display controller converts the digital
video signal into individual analog video signals corresponding to
red, green, and blue primaries. The display controller transmits
the analog signals to red, green, and blue video amplifiers which
amplify the video signals, producing red, green, and blue beam
currents which drive the respective cathodes of a cathode ray
tube.
Current samplers are coupled to the outputs of the respective video
amplifiers to sample the individual beam currents. The output of
each current sampler is fed into an analog-to-digital converter
which converts the red, green, and blue analog beam current
measurements into digital values. The digital values are read by
the internal processor at periodic intervals, preferably initiated
by a timer, and stored in a non-volatile memory location.
Red, green, and blue correction factors are calculated for both
luminous efficiency degradation and for deviations in hue, based on
the stored sum-total beam current measurements in combination with
empirically-derived test data. These correction factors are used to
calculate corrected tristimulus values X, Y, and Z for red, green,
and blue. The corrected tristimulus values X, Y, and Z are then
used to calculate the amount of each respective beam current
necessary to compensate for color degradation due to aging of the
cathode ray tube. Finally, the respective gains of the red, green,
and blue video amplifiers are adjusted to achieve the beam current
necessary to compensate for color degradation in the CRT.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a prior art system for measuring
and adjusting the color content of a video display;
FIG. 2 is a schematic diagram of the present invention showing a
system for compensating for the effects of aging of phosphors and
the faceplate upon color accuracy in a cathode ray tube;
FIG. 3 is a flowchart of process steps for initializing a system
during manufacture according to the present invention;
FIG. 4 is a flowchart of process steps for periodically measuring
and storing red, green and blue primary sum-total beam current
measurements;
FIG. 5 is a graph of a luminous efficiency degradation curve for
red and green hues based on empirical test data from Sony
Trinitron.RTM. video monitors;
FIG. 6 is a graph displaying the results of empirical test data
which depicts the more complex degradation curve of blue luminous
efficiency degradation in Sony Trinitron.RTM. video monitors;
FIG. 7 is a flowchart of steps for calculating corrected
tristimulus values X, Y, and Z for use in determining the
adjustment of beam current necessary to compensate for aging of the
phosphors and faceplate; and
FIG. 8 is a flowchart of the process utilized in the present
invention for calibration of a reference white screen display by
adjusting beam currents based on corrected tristimulus values.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 shows an improved system, according to the present
invention, for compensating for the effects of aging of phosphors
34 and a faceplate 33 upon color accuracy in a cathode ray tube
(CRT) 36. The invention provides a CRT 36 with a timer 21, a
measuring circuit 2 and a compensation circuit 5. In the preferred
embodiment, a CRT 36 monitor cabinet 38 houses the components of
the measuring circuit 2. This assures that the beam current history
accompanies the CRT 36, rather than possibly becoming separated if
stored in a host processor.
Host processor 10 is preferably a single-chip integrated circuit
microprocessor based in an Apple Macintosh.RTM. computer
manufactured by Apple Computer, Inc. of Cupertino, Calif. However,
host processor 10 may be any computing processor, for example, a
general purpose computer.
Host processor 10 communicates with internal processor 23 via a
first digital data bus 11. Non-volatile memory 24, which may be an
Electrically Erasable Programmable Read-Only Memory (EEPROM) or any
other suitable non-volatile memory, communicates with internal
processor 23 via a second digital data bus 39.
Internal processor 23 provides digital signals via a third digital
data bus 37 to a display controller 30 wherein the received digital
signals are conventionally converted into three discrete analog
signals 28, which drive respective red, green and blue video
amplifiers 47. Display controller 30 consists of a
digital-to-analog converter and appropriate buffers for maintaining
the voltage levels required to drive the video amplifiers 47 of a
video display. The three video signals 28 are applied to respective
video amplifiers 47, which are characterized typically by a high
input impedance and an output impedance sufficiently low to drive a
CRT 36. Video signals 28 are amplified to generate red, green, and
blue beam currents on lines 32 which drive the respective cathodes
35 of CRT 36. CRT 36 is a conventional color cathode ray tube with
red, green, and blue phosphors 34 deposited on the interior surface
of the tube's face. Further, CRT 36 includes a glass faceplate 33
on the tube's exterior face. CRT 36 and its associated electrical
components are preferably contained within video monitor cabinet
38, leaving only the faceplate 33 of CRT 36 exposed for viewing the
displayed images.
The magnitudes of the beam currents on each of lines 32 are sensed
by respective current samplers 54 to yield corresponding analog
beam current measurements and provided to Analog-to Digital
Converter (ADC) 50. Current samplers 54 are well known in the art
and may consist, for example, of current mirrors or networks of
passive electronic components. The individual analog beam current
samples are converted by ADC 50 to digital beam current
measurements, which are then communicated along a fourth digital
data bus 49 to internal processor 23.
A conventional timer 21 is connected to internal processor 23, and
at periodic intervals provides clock signals to initiate red,
green, and blue beam current measurements to be performed by
internal processor 23.
FIG. 3 is a flowchart of preliminary steps which typically occur
during manufacture of the CRT. In step 55, a beam current is
generated for each color. In step 56, initial values X, Y, and Z
are divided by the corresponding color beam current value to
produce normalized initial tristimulus values X, Y, and Z for each
color channel. Next, in step 57, the normalized initial tristimulus
values are converted to equivalent color coordinates U', v', and
normalized Y for each color. Then, in step 58, a non-volatile
memory 24 is provided for storing sum-total beam current
measurements for each color. In step 59, values of zero are stored
into non-volatile memory 24 for each color.
FIG. 4 is a flowchart describing steps for periodic storage in
non-volatile memory 24 of sum-total beam current measurements 31
for the outputs of the red, green, and blue video amplifiers 47.
The FIG. 4 procedure begins in step 61, where timer 21 clocks
internal processor 23 to generate a command to measure beam
currents. Next, in step 62, internal processor 23 reads the digital
output of ADC 50 to measure each color beam current. Then, in step
63, internal processor 23 reads the sum-total beam current
measurement for each color previously stored in memory 24. In step
64, internal processor 23 adds the beam current measurement for
each color to the contents of the non-volatile memory 24 for each
color to obtain sum-total red beam current measurements 31 for each
color. In step 66, internal processor 23 replaces the contents of
non-volatile storage memory 24 with the newly-calculated sum-total
beam current measurements 31 each color.
FIG. 5 is a graph of empirically-derived data for red and green
luminous efficiency degradation of Trinitron.RTM. display monitors.
The FIG. 5 graph conforms to luminous efficiency degradation
factors predicted using a mathematical equation known as Pfahnl's
Law. Pfahnl's Law yields acceptable luminous efficiency degradation
factors for red and green, expressed as relative efficiency .eta..
These red and green luminous efficiency degradation factors may be
calculated by the formula ##EQU1## where .eta..sub.(start) is the
initial efficiency for a red or a green phosphor, C is the phosphor
burn sensitivity parameter, and N is the dose of electrons
deposited on the phosphors, expressed in Coulombs per square
centimeter.
FIG. 6 is a graph of empirically-derived data for blue luminous
efficiency degradation in Trinitron.RTM. display monitors. The
graph in FIG. 6, however, does not conform to luminance degradation
factors predicted using Pfahnl's Law. Instead, a more complex
degradation curve was empirically observed for the blue phosphors.
The rapid early degradation of blue phosphors illustrated in FIG. 6
causes a significant chromaticity shift in display monitors early
in their lifetimes. The inventor of the present invention has
improved the degradation curve yielded by Pfanhl's Law by adding a
second degradation curve having a shorter time constant. Based on
the empirical test data, the luminous efficiency degradation factor
of the blue phosphors may be expressed as relative efficiency
.eta..sub.Blue, according to formula ##EQU2## where
.eta..sub.Blue(start) is the initial efficiency of blue phosphor, C
is the phosphor burn sensitivity parameter, N is the dose of
electrons deposited on the phosphor expressed in Coulombs per
square centimeter, S is the shape of the blue component second
degradation curve, and L is the amplitude of the blue component
second degradation curve.
The present invention uses hue deviation factors in combination
with the luminous efficiency degradation factors to calculate
corrected tristimulus values used in compensating for aging of the
CRT 36. A linear approximation of hue deviation factors .DELTA.u'
and .DELTA.v', for each primary color, was found to adequately
describe the change in color coordinates observed experimentally.
The formulae used to calculate hue deviation factors .DELTA.u' and
.DELTA.v' are .DELTA.u'=ND.sub.u', and .DELTA.v'=ND.sub.v' where
.DELTA.u' and .DELTA.v' are the shift in color coordinates of the
phosphor emission as observed through the faceplate 33, N is the
total charge deposited on the phosphors 34, and D.sub.u' and
D.sub.v' are the hue degradation factors for the u' and v' axes,
respectively. This correction is applied to each phosphor
independently, using each phosphor's respective accumulated dosage,
as stored in non-volatile memory 24.
FIG. 7 is a flowchart detailing steps for applying luminous
efficiency degradation values .eta. and hue deviation factors
.DELTA.u' and .DELTA.v', to the normalized initial tristimulus
values X, Y, and Z of a CRT, to obtain corrected tristimulus values
for adjusting the beam current 32 to compensate for CRT aging. The
procedure begins in step 74 where a luminous efficiency degradation
factor .eta. and hue deviation factors .DELTA.u' and .DELTA.v' are
calculated for each color according to the prior discussion of
FIGS. 5 and 6. In step 75, normalized initial tristimulus values
for each color are converted into corresponding u', v', and
normalized initial Y for each color. In step 76, u' is added with
.DELTA.u' to yield a corrected u' and v' is added with .DELTA.v' to
yield corrected v' for each color. In step 78, normalized initial
tristimulus value Y for each color is multiplied by luminous
efficiency degradation factor .eta. for each color to yield
normalized corrected tristimulus value Y for each color. In step
80, corrected color coordinates u' and v' for each color, along
with the normalized corrected tristimulus value Y for each color
are re-converted back into normalized corrected tristimulus values
X, Y, and Z for each color, yielding three sets of normalized
corrected tristimulus values to be utilized when calculating beam
current adjustments to compensate for aging of CRT 36.
FIG. 8 is a flowchart of steps for adjusting the beam currents 32
to calibrate a reference white screen display to compensated for
the aging of CRT 36. First, in step 94 host processor 10 calculates
the amount of beam current necessary to compensate for normalized
corrected tristimulus values X, Y, and Z for each color. In step
96, internal processor 23 generates a reference white screen on CRT
36. Finally, in step 98, the gain of the video amplifier 47 for
each color is adjusted to produce the beam current on line 32
necessary to compensate for the aging of CRT 36. Completion of the
FIG. 8 calibration procedure achieves the present invention's goal
of accurate compensation for aging of the phosphors 34 and
faceplate 33 in CRT 36.
The invention has been explained above with reference to a
preferred embodiment. Other embodiments will be apparent to those
skilled in the art, in light of this disclosure. For example,
instead of adjusting the gains of the video amplifiers 47, a
similar compensation could be effected on beam currents 32 by
causing display controller 30 to adjust the magnitude of the video
signals 28 input into video amplifiers 47. Further, other display
devices such as plasma displays and light emitting diodes having
various signal-receiving electrodes may be used in place of CRTs.
Therefore, these and other variations upon, and modifications to,
the preferred embodiments are intended to be covered by the present
invention, which is limited only by the appended claims.
* * * * *